Update graphite-nozzle to use Go 1.6
Update graphite-nozzle to use Go 1.6

The current version of the Go buildpack no longer supports Go 1.4 which
the graphite-nozzle app was pinned to. This updates the app to a version
which uses Go 1.6.

This also pins the buildpack in the application manifest to avoid having
to use buildpack detection, which will speed up the deployment slightly.

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